A) Field of the Invention
The present invention relates to a micro lens array suitable for coupling to an optical component such as an optical fiber array and to a method of manufacturing such a micro lens array.
B) Description of the Related Art
A conventional micro lens array such as shown in FIG. 102 is known. For example, refer to Japanese Patent Laid-open Publication No. 9-90162. FIGS. 99 to 101 are diagrams illustrating a method of manufacturing a micro lens array.
In a process shown in FIG. 99, on one principal surface of a silicon substrate 3 having a thickness of 500 μm, a quartz glass layer 4 having a thickness of 50 μm is formed and thereafter resist layers 5a to 5c corresponding to a desired lens pattern are formed on the quartz glass layer 4 and shaped in a convex spherical shape by photolithography and heat treatment.
In a process shown in FIG. 100, the resist layers 5a to 5c and quartz glass layer 4 are etched by reactive ion etching (RIE) to transfer the resist pattern of the resist layers 5a to 5c to the upper surface layer of the quartz glass layer 4 to form convex lenses 4a to 4c corresponding to the resist layers 5a to 5c. The diameter of each convex lens may be 60 μm. Thereafter, on the other principal surface of the substrate 3, a resist layer 6 having holes 6a to 6c for forming contact holes are formed by photolithography.
In a process shown in FIG. 101, by using the resist layer 6 as a mask, contact holes 3a to 3c facing the convex lenses 4a to 4c are formed in the silicon substrate 3 by dry etching. Each contact hole may have a depth of 500 μm and a diameter of 125 μm (corresponding to the diameter of an optical fiber).
FIG. 102 shows an optical fiber 7 inserted into the contact hole 3a of the micro lens array shown in FIG. 101. Since the depth of the contact hole 3a is twice or more the diameter of the contact hole 3a, the optical fiber 7 can be held reliably in the contact hole 3a. The center axis of the convex lens 4a is coincident with the center axis of the contact hole 3a, and the focal length of the convex lens 4a is coincident with the distance between the apex of the convex lens 4a and the bottom of the contact hole 3a. Therefore, if the optical fiber 7 is inserted into the contact hole 3a so as to make the front end of the optical fiber 7 contact the bottom of the contact hole 3a, the focal point of the convex lens 4a can be theoretically made coincident with the center point of the front end of the optical fiber.
According to the above-described prior art, in the processes shown in FIGS. 100 and 101, it is practically difficult to form the contact hole 3a so as to match the center axis and focal length of the convex lens 4a. In order to properly collimate light from the optical fiber 7 at the convex lens 4a, it is necessary to adjust the position of the optical fiber by entering light into the fiber each time each fiber is inserted into the contact hole. This is a time-consuming and cumbersome work.
Since the micro lens array with optical fiber contact holes is formed by working a composite substrate made of the silicon substrate 3 formed with the quartz glass layer 4, it is not possible to form convex lenses on a surface of the quartz glass layer 4 opposite to the surface of the quartz glass layer 4 formed with the convex lenses 4a to 4c or to perform oblique polishing of the opposite surface. In other words, it is necessary to use both surfaces of the quartz glass layer 4 in order to form a micro lens array of a both-side convex lens type or a micro lens array having a polished surface for suppressing reflection return light. In this case, the optical fiber contact holes cannot be formed by using the silicon substrate 3 and coupling to an optical fiber array is impossible.